Throughout much of recorded history, lightning was feared and often seen as a form of punishment (though also sometimes associated with fertility - eg. birth of twins).

“symbols based on the concept of fire are among the oldest pictorial representations of lightning.”  (see Ref. (1))

“In many ethnological representations from prehistoric times lightning is depicted …. as a stone falling from heaven or a stone axe hurled from the skies.”  (Ref(1)).  Stones falling from the skies seems to be a very widespread association. 

“French peasants carry a “pierre de tonnerre” (thunder stone) in their pockets to ward off lightning during thunderstorms” ref (1).  Having lived and worked in France for a short time I was curious about this custom (and  ready to buy one on eBay or during my next trip to France). 

When I taught this course in Spring 2011 I wasn't able to find much additional information.  This year was different.  Wiktionnaire (the French version of Wiktionary) defines pierre de tonnerre as: "Boule de Marcassite" (ball of marcasite) and "Hache polie" (polished axe) and gives a couple of synonyms: "boule de tonnerre" and "pierre de foudre" (lightning stone).   Wiktionnaire goes on to explain that a thunder stone is "supposee naitre de la foudre frappant le sol" (thought to be created by lightning striking the soil).  The balls of marcasite are frequently found in the chalky soils of the Champagne region.

Marcasite is a mineral and is sometimes called white iron pyrite.   Here are a couple of pictures.

Boule de tonnerre (source)	A marcasite geode cut in half and polished (source)

A ball of marcasite has an unusual appearance and must be heavy because iron pyrite is dense (about 4.9 g/cm³).  At this point I was beginning to wonder how could a ball of iron pyrite be produced by lightning striking chalky soil? 

Here's an image of a polished neolithic axe (source)

Clearly this wasn't produced by lightning.  But an object like this would qualify as a "pierre de tonnerre" and was thought to offer protection from lightning. 

Some additional research led to a site that mentioned an "oeuf de tonnerre" (if you click on the link you'll find the picture below and many more beautiful examples like it, oeuf by the way is the French word for egg).  These "thunder eggs" are produced in cooling lava as best I can tell.

The site explains the origin of the term thunderegg:
Selon la légende Amérindienne ,lorsque les esprits de la tempête qui vivaient sur les sommets enneigés du Wy'east (mont Hood) et le Mont Jefferson dans l'orégon, se mettaient en colère les uns contres les autres, ils se lançaient des boulets de rochers, les dieux trouvaient ces armes dans les nids des oiseaux tonnerre (thunderbirds) d'ou la dénomination Thundereggs.

A rough translation: According to American Indian legend, when the storm spirits that lived on the snow covered summits of Wy'east (Mt. Hood) and Mt. Jefferson (in Oregon) would throw balls of rock at each other when they became angry.  The gods founds these weapons in the nests of thunderbirds hence the term thunderegg.

A final site explains that a pierre de tonnerre is often just a fragment of a meteorite.

So after a somewhat time consuming, but interesting, detour I concluded that none of these objects was actually produced by lightning.  Rather, at one time, because of their unusual appearance and weight were thought to be produced by lightning or, in the case of a meteorite, observed to fall from the sky.  They were something of value because the belief was that lightning would never strike the same location twice and thus either carrying one of these stones or putting it on the threshold of a home would protect the bearer or the structure from lightning.

Franklin actively experimented with electricity for only a few years starting in the 1740s & continuing into the early 1750s.

There were many people in the American Colonies and Europe that were interested in and actively studying electricity at the time.  Franklin emulated Newton & performed experiments to test his theories.

Electrostatic machines (friction machines) were in wide use.  Peter Collinson – unpaid London agent of the Philadelphia Library Company gave Franklin a glass-rubbing tube in late 1746.

Early machines often used some material (my guess would be silk) rubbing against a spinning glass sphere or a glass cylinder.  The glass would acquire one charge (positive charge I suspect), the rubbing surface the other polarity.  The charge could be drawn from the glass cylinder by a metal comb.

Here's a rough translation of the French phrase at the bottom of the figure: "I know, where is best found this almost magical virtue, wisely named electricity; young beauties it's in your eyes.")

“By being removed so far from the European centres of experimentation and discussion of electrical events.  Franklin was able to view his own observation with a freshness not encumbered by the earlier notions of others.  He therefore regarded an electrically undisturbed body as being under neutral charge or as in a state of electrical equilibrium." (Ref (2))

Franklin believed (correctly) that rubbing two materials together did not create electricity.  Rather, the rubbing somehow or another "grabbed onto" and separated charges that already were part of the neutral materials.  Material 1 might "tear" electrons from material 2.  Material 1 would become negatively charged and material 2 would be left positively charged.

Priestley explains this more clearly and succintly:

“Dr. Franklin had discovered … that the electric matter was not created but collected by friction, from the neighbouring non electric bodies.” Priestley was the first historian of electrical science.  His “History and Present State of Electricity” appeared in London in 1767.

A Van de Graaff generator (invented in 1929) is a more modern way of producing electric charge like was used in some of the early electrical experiments.  A Van de Graaff generator is an example of a friction machine and it is worth taking a moment to understand how they work.  A photo is show below at left and a sketch at right.

The lower roller (1) is made of plexiglas.  Friction between the belt and the plexiglas roller causes the roller to become positively charged (the inside surface of the belt is negatively charged).  We know this from the Triboelectric Series shown below. 

A strong electric field develops between the positively charged plexiglas roller and the comb of metal points connected to ground.  The electric field is strong enough to ionize the air and corona discharge "sprays" electrons onto the belt. This charge is then carried upward toward the top of the generator (3).  The upper metal comb draws off negative charge from the belt and it moves to the surface of the metal dome.

Friction between the belt and the upper roller (made of polyethylene) causes the upper roller to become negatively charged.  Corona discharge between the comb and the negatively charged roller adds additional negative charge to the dome.

If this explanation is correct (and I'm not entirely sure it is) we should find that the dome in this case is negatively charged.  Later in the course we'll use an electric field mill to determine whether that is really the case.

---

Here's a short description of triboelectric charging (from Wikipedia)

"The triboelectric effect (also known as triboelectric charging) is a type of contact electrification in which certain materials become electrically charged after they come into contact with another different material through friction. Rubbing glass with fur, or a comb through the hair, can build up triboelectricity. Most everyday static electricity is triboelectric. The polarity and strength of the charges produced differ according to the materials, surface roughness, temperature, strain, and other properties.

Thus, it is not very predictable, and only broad generalizations can be made. Amber, for example, can acquire an electric charge by contact and separation (or friction) with a material like wool. This property, first recorded by Thales of Miletus, suggested the word "electricity" (from William Gilbert's initial coinage, "electra"), from the Greek word for amber, ēlektron. Other examples of materials that can acquire a significant charge when rubbed together include glass rubbed with silk, and hard rubber rubbed with fur."

And here is list many materials and the charge they acquire (from the same article in Wikipedia).  This was on a handout distributed in class.

The Triboelectric Series

Positively charged (most charging at the top of the list)	No charge	Negatively charged (most charging at the top of the list
polyurethane foam Hair, oily skin nylon, dry skin glass acrylic, lucite leather rabbit's fur quartz mica lead cat's fur silk aluminum paper (small positive charge) cotton	wool steel	ebonite silicone rubber teflon silicon vinyl (PVC) polypropylene polyethylene (like Scotch tape) plastic wrap orlon sytrene (Styrofoam - polystyrene foam) polyester synthetic rubber acetate, rahyon gold, platinum brass, silver sulfur nickel, copper hard rubber resins rubber balloon polystyrene sealing wax amber wood (small negative charge)

This is what was used to determine the charging of the plexiglas and polyethylene rollers in the Van de Graaff generator

---

The Wimhurst generator was developed between 1800 and 1883 by a British inventor and is another method for separating and collecting charge.  Friction and triboelectric charging is not involved here.  Leyden jars are shown on the right and left (photo from Wikipedia)

Leyden Jar capacitors (invented independently in 1644 and 1645-46) were used to store electrical charges produced by the friction machines.  Franklin described a demonstration involving a dissectible leyden jar.  When assembled the two metal cups form a capacitor with a glass dielectric in between.  The leyden jar capacitor can be charged up and then disassembled.  No spark is observed when the two metal cups touch.  A spark is observed when the leyden jar is assembled and the inner and outer cups are connected.  Franklin believed (incorrectly) that it demonstrated that charge is stored on the dielectric that is between the two pieces of metal.

You'll find a short description (and some explanation) of the experiment on Wikipedia.  A portion is reproduced below

A popular but misleading demonstration with a Leyden jar involves taking one apart after it has been charged and showing that the charge is stored on the dielectric not the plates. The first documented instance of this demonstration is in a 1749 letter by Benjamin Franklin.  Franklin designed a "dissectible" leyden jar, shown below, which was widely used in demonstrations.

The jar in the demonstration is constructed out of a glass cup nested between two fairly snugly fitting metal cups. When the jar is charged with a high voltage and carefully dismantled, it is discovered that all the parts may be freely handled without discharging the jar. If the pieces are re-assembled, a large spark may still be obtained.

When not properly explained, this demonstration promotes the myth that capacitors store their charge inside their dielectric. This erroneous theory, due to Franklin, was taught throughout the 1800s, and is still sometimes encountered. However this phenomenon is a special effect caused by the high voltage on the Leyden jar.  In the dissectible Leyden jar, charge is transferred to the surface of the glass cup by corona discharge when the jar is disassembled; this is the source of the residual charge after the jar is reassembled. Handling the cup while disassembled does not provide enough contact to remove all the surface charge.

Here's a pretty good video of the demonstration.

---

References
(1) V.A. Rakov, "Lightning Makes Glass," 29th Annual Conference of the Glass Art Society, Tampa, Florida, 1999.